The manufacture of semiconductors, as in LCD technology, is generally a photolithographic process in which a solution of a resist material is applied as a thin coating over the substrate, heated to remove the solvent, and then subsequently exposed to electromagnetic radiation in an image-wise fashion, typically through a mask. Exposed areas of the photoresist are transformed chemically and/or physically to pattern a latent image that can then be developed by standard methods into a three-dimensional image. The resist film that remains after development serves as a protective mask, allowing the resist image to be transferred onto the substrate by etching or similar processes. Typically, the remaining resist film is stripped after the etching step, leaving an image of the desired circuit in the substrate. The process, together with other deposition processes, may be repeated many times to fabricate three-dimensional semiconductor devices. A passivation layer is often applied to protect the circuitry of the semiconductor devices from moisture and contamination.
In addition to being useful in LCDs, passivation layers may also be useful in applications such as organic electroluminescent displays (OELDs). In an OELD, a conductive transparent anode layer, a hole injection layer, a hole transport layer, an organic electroluminescent layer, an electron transport layer and a cathode layer are stacked successively on a transparent substrate, such as glass, quartz or the like. Because the organic material is sensitive to oxidation, moisture and contamination, the OELD also needs a passivation layer. See U.S. 2003/0003225, incorporated herein by reference.
Light emitting diodes (LEDs) are p-n junction devices, in which the electrons cross a forward-biased junction from the n- to the p-type material. The electron-hole recombination process produces some photons in the visible via electroluminescence, by which an exposed semiconductor surface can then emit light. A passivation layer is also used in LEDs. See U.S. 2004/021415, and Kho, S et al (2002) JAP J APPL PHYS, PART 2: LETTERS 41:1336-1338, both of which are incorporated herein by reference.
A passivation layer based on silicon dioxide or silicon nitride, also called a hard coat, may be deposited to cover and insulate the surface. See Hong, W S, et al. (2003) MAT. RES. SOC. SYMP. PROC. 762: 265-270; U.S. 2003/143319. The passivation layer may be variously termed a “gate insulating film” when covering the gate electrodes, or a “channel protection film” when covering the silicon layers. The passivation layer protects the gates and channels from moisture, contamination and/or mechanical damage. However, such hard coats are difficult and expensive to apply, typically requiring high-vacuum equipment and vapor deposition methods.
Various copolymer products for photoresist compositions that may be used as passivation coatings have been described in Thompson, L F, Willson, C G and Bowden, M J (1994) INTRODUCTION TO MICROLITHOGRAPHY, 2nd Ed., American Chemical Society, Washington, D.C. In general, a photoresist composition may comprise a film-forming polymer, which may be photoactive, and a photosensitive composition, such as a photoacid generator, that includes one or more photoactive components. Upon exposure to electromagnetic radiation, e.g., visible (VIS) and ultraviolet (UV) light, the photoactive components of the copolymer can change various electromagnetic, physical or chemical characteristics of the photoresist composition, which include the rheological state, solubility, surface characteristics, refractive index and color, etc., as described in Thompson et al. (supra).
In some applications, it would be desirable to be able to image very fine features (at the submicron level) in the passivation layer. This requires use of electromagnetic radiation in the far or extreme UV range and a photoresist composition that is suitable for use at such wavelengths. The opacity of traditional aromatic-based photoresist materials precludes their use at 193 nm and shorter wavelengths, especially, in single-layer schemes.
Some photoresist compositions suitable for imaging at 193 nm are known, such as materials based on aliphatic polymers and dissolution inhibitors. See, e.g., Meagley, R P et al., CHEM. COMM. 1587 (1999); Houlihan, F M et al. (1997) MACROMOLECULES 30: 6517-6534; Wallow, T et al. (1997) SPIE 2724: 355-364; and Houlihan, F M et al. (1997) J PHOTOPOLYMER SCI & TECHNOL 10(3): 511-520, disclosing photoresist compositions comprising cycloolefin/maleic anhydride alternating copolymers useful for imaging of semiconductors at 193 nm. See also, Okoroanyanwu, O et al. (1997) SPIE 3049: 92-103; Allen, R et al. SPIE 2724: 334-343; and Niu, J and Frechet, J (1998) ANGEW CHEM INT ED 37(5): 667-670, focusing on 193 nm resists. For optical lithography at 157 nm, the incorporation of fluorine into polymers has begun to provide suitably transparent resist materials. See Brodsky et al., J. VA. SCI. TECHNOL. B 18:3396 (2000).
Although photoresist layers are typically removed after serving their protective role during the etching process, passivation layers become a permanent part of the semiconductor or display device. For this reason, materials used in the passivation layer must be mechanically robust and have good electrical insulating properties.
For the manufacture of various display and other electronic devices, there remains a need for a composition that serves as a photoresist and a passivation layer for use at 356 nm or lower wavelengths, which possesses high transparency in the visible (e.g., about 400-900 nm) and other valuable properties, especially for multiple-layer schemes. In particular, the passivation resist composition should be photoimageable, thermosettable and have a low dielectric constant that assures insulation. Photoimageability gives the resist patternability; thermosettability provides robustness and allows the resist to function as a retained dielectric layer between conducting layers.